1. Field of the Invention
The present invention relates to a toner for development of an electrostatic image that can be used in image forming methods such as electrophotography, electrostatic recording and the like, and to an image forming method using the same.
2. Description of Related Art
In recent years, as image forming apparatuses such as electrophotographic copier and the like have become widely used, uses thereof have been extended to various applications and there are severe needs for higher image quality. In copying of images such as general documents, books and the like, even the microscopic characters are required to be very finely and faithfully reproduced without being crushed or broken. Where a latent image on a photoreceptor of an image forming apparatus to be reproduced is a line image having a line width of 100 μm or smaller, an ordinary plain-paper copier can not achieve a sufficient sharpness as it has a poor thin line reproducibility.
In image forming apparatuses such as electrophotographic printers and the like which use digital image signals, a latent image is formed of a pattern of dots having a certain potential. Solid areas, halftone areas and lightly shaded areas are formed by changing the dot density. However, there is a problem that toner particles can not precisely adhere to the dots and the particles lie off the outline of the dots. Because of this, the gradation of a toner image can not correspond to the dot density ratio between black and white portions in a digital latent image. Where the dot size is reduced to improve the resolution for better image quality, reproduction of a latent image formed of microscopic dots become more difficult and there is a tendency that a image with poor resolution and gradation as well as reduced sharpness is formed.
Furthermore, though the above image forming apparatuses can achieve satisfactory image quality in the beginning, there are cases where the image quality degrades as the number of copies or prints increases. It is believed that this happens because toner particles which are easier to be developed are consumed first and toner particles which are difficult to be developed accumulate and remain in a developer unit as the copying or printing continues.
In order to improve the image quality, there has been proposed a number of developers. In Japanese Unexamined Patent Publication No. SHO 51(1976)-3244, for example, a toner with a limited particle diameter distribution is proposed for the improvement of image quality. In this patent publication, it is described that the toner in which not less than about 60 percent by number of toner particles have a particle diameter of 8 μm to 12 μm is most desirable. However, the toner particles in this particle diameter range are relatively coarse, and according to the study made by the present inventors, it is still difficult to make the toner of this particle sizes to closely adhere to a latent image. Furthermore, the toner of this patent publication is characterized by comprising toner particles having an average particle diameter of 5 μm or smaller in an amount of not more than 30 percent by number and toner particles having an average particle diameter of 20 μm or smaller in an amount of not more than 5 percent by number. This wide range of particle diameter distribution tends to reduce the uniformity of the toner. Therefore, in order to form a sharp image using the toner of the aforementioned patent publication comprising coarse toner particles and having a wide particle diameter distribution range, the toner particles need to be thickly overlaid to fill space between the particles so that the apparent image density increases. This results in an increase in toner consumption necessary for obtaining a predetermined image density.
In Japanese Unexamined Patent Publication No. SHO 54(1979)-72054, there is proposed a toner having narrower particle diameter distribution than that of the toner of aforementioned patent publication. However, middle-weight particles have a coarse particle diameter of 8.5 μm to 11.0 μm and further improvement needs to be made to obtain a high-resolution toner.
In Japanese Unexamined Patent Publication No. SHO 58(1983)-129437, there is proposed a toner in which the average particle diameter of toner particles is 6 μm to 10 μm and particles of 5 μm to 8 μm in diameter occupy the largest number. Since the number of toner particles with a particle diameter of 5 μm or smaller is as small as 15 percent by number, images with reduced sharpness tend to be formed.
In Japanese Patent No. 2763318, a toner contains toner particles having a particle diameter of 5 μm or smaller in an amount of 17 to 60 percent by number so as to inhibit a change in toner properties associated with long use thereof. Though the toner of this patent withstands use for about hundred thousand copies or prints, recent machines aiming for longer machine life require durability to withstand use for about five hundred thousand copies or prints, and when this toner is used in such machines, toner particles with poor developability accumulate and remain in a developer unit, and thus, the durability is not sufficient.
According to the study made by the present inventors, it is found that toner particles with particle diameter of 4 μm to 5.04 μm can clearly reproduce the outline of a latent image and play an important role in making toner to closely adhere to the entire latent image.
Particularly, since lines of electric force are concentrated at the edge (outline) of the image and thereby the edge of the image is higher in field intensity than the inner portion of the image, the sharpness of the image depends on the quality of toner particles concentrated in this area. It is found by the present inventors that the toner particles with a particle diameter of 4 μm to 5.04 μm are effective in solving this problem related to image sharpness. Furthermore, it is found that toner particles with a particle diameter smaller than 4 μm accelerate degradation of developers, and thus, reduction of the number of such particles is a main factor for achieving longer toner life.